Interbus Communication

Interbus is a system for serial data transmission which transfers data between different systems/ computers and controllers and which is connected with actuators and sensors, e.g. for temperature control on a machine.
Entire production equipment and higher-level systems can be connected with each other quickly, safe, and easily. All subscribers are connected actively by the ring system.

The Interbus system was developed by Phoenix Contact and has been available since 1987. Since its introduction at the Hanover Messe, it is one of the leading fieldbus systems that meets the European standard. It is an open network system and as such, it is distributed by several manufacturers.

How does Interbus work?

The serial Interbus technology was revolutionary and has facilitated the networking of enterprises considerably. It offers fast, easy handled communication and is characterised by low susceptibility. 512 subscribers (max.) can participate, and the last subscriber closes the ring. This way, the ring structure is always assured.

This always remains user-friendly, as subscribers can be added or removed at any time. Closing of the loop is assured by the loop-back bit combination, which passes once through the loop and signals to the master that the circle is closed and that everything functions correctly.

As each participant is connected by two bus terminals, a tree structure physically results. This permits incoming and outgoing signals that are processed by the master. Actuators and sensors are networked per loop, so that flexible, stable, and efficient networks are possible.

Today, 17 million devices are installed worldwide and as such, the technology has been accepted widely. Currently over 600 enterprises participate in converting Interbus to control systems in order to keep it up to date.

Furthermore, fieldbus technologies are changing, and the tendency is towards Ethernet technology. For this reason, work has been done and is being done to connect the two technologies with each other in order to permit optimum modern networking.

Advantages of the Interbus

Interbus runs on a transmission rate of 500 kbps. Branch bus terminals enable a sub-network, and coupling elements prevent faults from spreading from one network to another. This contributes to the stability of the system. The local buses branch from the bus terminal which is connected to the remote bus, and in doing so they divide the network. These sections then can be switched on and off independently of each other. Addressing is done automatically, so that time-consuming calibration is not required.

This also facilitates maintenance and promotes smooth switching to the serial bus system. Errors during installation are a main cause of the trouble that occurs in networks. This is prevented through automatic installation and immediate diagnosis of the system.

Elements of the Interbus

The secret of a functioning network is the communication of its components. For this purpose, the remote bus, which assures the power supply and permits a distance of 400 m between two participants, is an important element of the Interbus. Bus terminals connecting a branch network with the main network are connected to the local bus. It can be switched off separately, but does not permit any further branching.

The Interbus loop, which integrates actuators and sensors into the network, allows a distance of up to 20 m between two participants. At this point, the ring structure now also becomes physical. All of these elements function as slaves with regard to a master connected to a remote bus (this is also known as a connection group).

Diagnosis with CDM

CMD was developed for control and diagnosis (and stands for Configuration Monitoring Diagnostic). It is useful in all areas of Interbus operation: In the process planning phase, CMD can be used to configure the equipment with all program addresses. During operation, the connection group can be maintained and operated using the CMD. In addition, the sub-installations can be checked. The data gathered by the CMD are stored centrally and in doing so, they are available to all participants. As such, CMD clearly facilitates the work with the Interbus and makes smooth operation possible.

In the case of a fault with a subscriber or a short-circuit, the network functions up to the fault location. This makes it easy to locate the fault, and makes quick elimination of the problem possible. In the case of transmission pauses, the subscribers are placed into a safe “idle state” condition. This way, that the subscribers are also placed into this safe “idle state” condition in the case of a fault, so that a fault does not cause permanent damage.

However, the master can maintain the ring so that the network continues normally, whilst the problem of the failed subscriber is removed. Furthermore, Interbus offers the statistical evaluation of faults and problems. These can be used to recognize and eliminate wear and tear phenomena before a larger problem occurs. A diagnosis display is installed on the remote bus and on the computer. The system can be checked with CMD. All of these elements make system maintenance easy and a long service life possible.


The term Interbus characterizes a fieldbus system that is designed for broad base use in companies that want to profit from use of automation technology. Here, the application covers various application areas from the sensor and actuator level in process automation through to the monitoring computer. Interbus was developed in 1983 by Phoenix Contag as specifications for a fieldbus that was suitable for industry.

Advantages of the Use of Fieldbuses

Companies engaged in manufacturing are confronted with the need for automated processes, just like other sectors of the economy. It is necessary to create data exchange between the machines so that the processes in a production facility can run by themselves. However, this form of communication places high requirements on the technologies, and these have to be met with fieldbuses.

Fieldbuses have the advantage that the costs of wiring all equipment are kept as low as possible. Here, in comparison with conventional wiring which is characterized by a large number of input and output points, the work comfort is increased. On the other hand, serial networking via the fieldbuses has the appeal of flexibility and cost reduction. When Interbus is used, there is only a single fieldbus cable that can be used for transmitting all kinds of data in any formats and quantities. At the same time, the type of automation equipment does not play an important role.

The Characteristics of Interbus

The Interbus is generally considered to be an optimal technology in the production installations of the automobile industry. This opinion about the benefits of using the fieldbus system is based on several recognized advantages. In this way,

  • start-up can take place quickly and without major problems,
  • There are high-quality, yet still simple diagnosis tools in the system.
  • The data throughput is high in spite of a low clock speed, and
  • there are many possibilities for connections via a light conductor, including functions such as the diagnosis of the route, error messages and overlaid controls.

The Interbus also includes

  • cost reduction using an integrated bus coupler which operates via light conductors,
  • the galvanic isolation of individual installation parts as well as the absolute insensitivity of the route with regard to coupling / EMV interference.

The Technology employed within the Interbus

From the point of topology, the operation of the fieldbus system is based on an active ring-shaped structure. However, the forward and the back channel are joined in a connection cable. Furthermore, each subscriber has two connection terminals, so that a tree-like structure of the physical wiring results.
When a bus participant wants to close the ring structure of Interbus, it can bridge the respective outputs internally.

However, this only works when no other subscriber follows which would be cut off by this bridging. When branching occurs at the bus terminal, the next new branch is bound in the forward channel. This expands the ring topology. This arrangement has the advantage that when faults occur in the system (for example if a subscriber fails) the network still does not crash as the preceding subscriber simply corrects this gap by bridging his output. This way, the system can be kept operating until the failed subscriber is once again in the position to participate in the communication. The tree structure of the Interbus is composed of three hierarchical levels occurring in four types:

  • Remote bus – from the electrical point of view, this is an RS-485-specified interface where the energy supply is performed locally at the subscriber level. 400 m of cable (max.) can exist between the two participants. However, when copper lines are used, the possible distance increases to 13 km.• Installation remote bus – this is basically specified like the remote bus, but has a central energy supply.
  • Local bus – this branches via the bus terminals, i.e. the bus coupler, from the level of the remote bus. No further branching is possible. Here as well, the energy supply takes place centrally and the local bus can separated from the remote bus separately so that it is then switched off.
  • Interbus loop – Here, the distance between two participants is 20 cm to 20 m as a rule, and the max. distance can be 200 m. The Interbus loop has a two-wire interface, via which the energy is supplied centrally. The data for the bus here are modulated so that at this point, the subscribers can be disbanded the physical ring structure.

Within the arrangement of the subscribers, each one takes the position of a repeater. Then the wiring of the remote bus and the local bus can be used via electrical conductors or light conductors. Additionally, all subscribers act as slaves under a master connected to the remote bus. This is called a connection assembly group.